1. Field of the Invention
The present invention relates to a phase shift mask and a fabrication method thereof which are used in a semiconductor light exposing process.
2. Description of the Background Art
As the integration density of semiconductor devices has increased, a distance between elements of a transistor in devices, (e.g., a capacitor, a wire, etc.) has decreased. Therefore, a technique is required for forming a fine pattern needed for semiconductor device fabrication.
Generally, a light exposing mask used for a light exposure process for forming a photoresist film pattern is formed by coating a light shield film such as a chromium film, an aluminum film, etc. on an upper surface of a quartz substrate. A part of the light shield film is then etched using an ion beam etching method, etc. to form a light exposure pattern. However, when a light exposure mask which uses the above-described light shield pattern is used, it is impossible to form a fine pattern having a visibility limit which is slightly lower than that of a stepper light source.
Recently, phase shift masks have been studied and used for forming fine patterns. Phase shift masks are formed with a semi-transparent phase shift region and a transparent light transmitting region. The light transmitting region and phase shift region have different refractive indexes and transmittance with respect to an incident wave. When the light wave which is transmitted by the phase shift region reaches the wafer, the wave has a phase difference. Therefore, the phase shift mask has information concerning a light intensity and information concerning the phase, where the light exposure mask which uses the conventional light exposure film pattern only has information concerning the light intensity.
The wave which is transmitted by the phase shift region and the wave which is transmitted by the light transmitting region interfere due to their phase difference. As a result, the photoresist film pattern formed using the phase shift mask has visibility superior to the photoresist film pattern formed based on the light exposure mask, so that it facilitates forming a fine pattern.
When the pattern formed on the mask is arranged based on an anisotropic pitch, (i.e., when the size of each light transmitting region formed on the mask is significantly larger than the wavelength of the exposing light) a phase having a shape similar to the shape of the light transmitting region pattern is formed on an upper surface of the wafer. When the size of the light transmitting region pattern has feature size similar to the wavelength of the exposing light, a circular-shaped phase is formed irrespective of the shape of the light transmitting region pattern.
FIG. 1 illustrates a conventional phase shift mask. The phase shift mask includes a phase shift region 1 and a light transmitting region 2. As shown therein, a square-shaped light transmitting region 1 is arranged with an xe2x80x9cXxe2x80x9d pitch in a first direction and is arranged at a xe2x80x9cYxe2x80x9d pitch in a second direction. The phase shift region 1 is positioned among the thusly arranged light transmitting regions 2. The phase shift region 1 is formed of the same material over its entire area and has the same thickness, thereby obtaining the same transmittance.
FIG. 2A is a cross-sectional view taken along line II-IIxe2x80x2 of the phase shift mask in FIG. 1. FIG. 2B illustrates an amplitude and phase which are obtained when the wave 5, which is transmitted by each light transmitting region 2 of the phase shift mask of FIG. 2A, and the wave 6, which is transmitted by each phase shift region 1, reach the wafer. As shown therein, the wave 5 does not overlap itself and has a 180xc2x0 phase difference with respect to the wave 6. In addition, the amplitude of the wave 5 is larger than the amplitude of the wave 6.
Since the wave 5, which is transmitted by the light transmitting region 2 of FIG. 2A, and the wave 6, which is transmitted by the phase shift region 1, have a 180xc2x0 phase difference, the waves symmetrically interfere. As a result, as shown in FIG. 2C, a wave 7 is formed. The wave 7 forms an image having a large visibility on an upper surface of the wafer.
FIG. 3A is a cross-sectional view taken along line III-IIIxe2x80x2 of the phase shift mask in FIG. 1. As shown therein, the light transmitting region 2 is formed at a xe2x80x9cYxe2x80x9d pitch, and the value of xe2x80x9cYxe2x80x9d is smaller than the value of xe2x80x9cXxe2x80x9d.
FIG. 3B illustrates an amplitude and phase which are obtained when the wave 10, transmitted by each light transmitting region 2 of the phase shift mask of FIG. 3A, and the wave 11, transmitted each phase shift region 1, reach the wafer. As shown therein, the waves 10 overlap and have a 180xc2x0 phase difference with respect to the waves 11. The overlapping of the waves 10 increases as the value of xe2x80x9cYxe2x80x9d of the arrangement pitch of the light transmitting region 2 decreases. The amplitude of the wave 10 is larger than the amplitude of the wave 11.
Since the phase shift regions of the conventional phase shift mask all have the same transmittance, the amplitude of the wave 11 is the same as the amplitude of the wave 6, both of which are transmitted by the phase shift region 1.
Since the wave 10 and the wave 11 of FIG. 3B have a phase difference of 180xc2x0, the waves symmetrically interfere. The overlapping waves 10 and the waves 11 combine to form a wave 12 having a phase and amplitude as shown in FIG. 3C. The wave 12 formed by the above-described interference and overlapping phenomena forms a certain image on the wafer which has a decreased visibility. As the arrangement pitch xe2x80x9cYxe2x80x9d of the light transmitting region 2 is decreased, the size of the overlap which is formed by the waves 10 increases, and the visibility of the image formed on the wafer decreases.
FIG. 4 illustrates an image formed on the wafer during a light exposing process using the phase shift mask as shown in FIG. 1. As shown in FIG. 1, the phase shift mask includes a square light transmitting region 2 having a pitch of xe2x80x9cXxe2x80x9d in a first direction and a pitch of xe2x80x9cYxe2x80x9d in a second direction. As shown in FIG. 4, an elliptical image 15 is formed by such a mask on the upper surface of the wafer.
When the size of each light transmitting region has a value similar to that of the exposing light""s wavelength, a circular image is not formed. Rather, an elliptical shape image 15 is formed, because when the pitches xe2x80x9cXxe2x80x9d and xe2x80x9cYxe2x80x9d are different, the visibility of the image formed on the wafer is different in each direction. Therefore, the shape of the image formed on the wafer is expanded in the direction that the arrangement pitch between the light transmitting regions is narrow and overlap of transmitted waves occurs.
Therefore, as the arrangement pitch of the light transmitting region 2 is decreased, the shape of the image formed on the wafer changes. As the integration of the semiconductor device is increased, such distorting effect is increased.
In particular, the above-described shape variation is increased when the arrangement pitch between the light transmitting region patterns is below two times of the exposing light""s wavelength. When the wavelength of a light source is smaller than that of the arrangement pitch between the light transmitting region patterns, it is possible to overcome the above-described problems. However, using such a light source creates problems.
When the image formed on the wafer is varied, errors in the fabrication process increase, thereby decreasing the yield, and the reliability of the semiconductor device is decreased.
Accordingly, the present invention is directed to a phase shift mask and a fabrication method thereof that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
In accordance with the purpose of the invention, as embodied and broadly described, in one aspect the invention includes a phase shift mask, including: a plurality of light transmitting regions arranged at different pitches; and a plurality of phase shift regions having a plurality of different transmissivities and formed among the light transmitting regions.
In another aspect, the invention includes a phase shift mask, including: a plurality of light transmitting regions; and a plurality of phase shift regions formed near the light transmitting regions, each phase shift region having a refractive index different from a refractive index of the light transmitting regions and having a transmittance different from at least one other phase shift region.
In still another aspect, the invention includes a phase shift mask, including: a plurality of light transmitting regions arranged at different pitches in a plurality of directions; a plurality of first phase shift regions formed adjacent to the light transmitting regions in one direction among the directions, the first phase shift regions having a refractive index different from a refractive index of the light transmitting regions and having a first transmittance; and a plurality of second phase shift regions formed adjacent to the light transmitting regions in another direction among the directions, the second phase shift regions having a refractive index different from a refractive index of the light transmitting regions and having a second transmittance different from the first transmittance.
In yet another aspect, the invention includes a phase shift mask fabrication method, including: forming a film by coating a material having a different refractive index and transmittance from a substrate on an upper surface of the substrate; selectively varying a thickness of the film; and patterning the film.
Accordingly, the present invention advantageously provides a phase shift mask and a fabrication method thereof which are capable of increasing the yield of a semiconductor device fabrication and significantly enhancing the reliability of a semiconductor device by providing the same critical dimension of images formed on a wafer when a pattern is formed on a mask based on an anisotropic pitch arrangement.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description serve to explain the principles of the invention.